1. Field of the Invention
The present invention relates to a method and apparatus for applying microwaves to a nitrate solution such as a solution of uranyl nitrate, plutonium nitrate, a mixture thereof or the like, to heat such nitrate solution, so that evaporation, concentration and denitration of the nitrate solution are conducted to produce a denitrated product.
2. Description of the Prior Art
Such a denitration technique employing the microwave heating is effectively employed, particularly in producing oxide powder for nuclear fuel pellets from the above-mentioned nitrate solution obtained in a reprocessing process of a spent nuclear fuel.
Hitherto, in conducting the evaporation, concentration and denitration of the nitrate solution by microwave heating as shown in FIG. 5, a cylindrical heating vessel 1 in which is received a substance to be treated, i.e. a nitrate solution S, is placed in an oven 2 which usually has a rectangular shape and to which microwaves are applied, and then the heating vessel 1 is horizontally rotated by means of a rotary unit 3 while heated. This is a conventional method. In the drawing, the reference numeral 4 denotes a waveguide tube and 5 denotes a gas discharging tube.
According to the application of microwaves to the solution which is the substance to be treated, such substance absorbs the microwaves so that the temperature of the substance increases. FIG. 6 is a graph showing an example of the change in temperature of the substance being treated in the case where the substance is a uranyl nitrate solution. As shown in this graph, the solution begins to boil when its temperature reaches 100 to 120 C. (at point A). Although the temperature of the solution is kept substantially constant while it is boiled and evaporated, the solution is concentrated to become a nitrate (UO.sub.2 (NO.sub.3).sub.2.H.sub.2 O) as a result of continued evaporation thereof (at point B). The thus produced nitrate is then gradually heated over time up to 300 C. (at point C). During this heating, the nitrate eliminates its water molecules and discharges No.sub.x gases produced in decomposition of the nitrate radical, so that it is converted into an oxide (UO.sub.3) which is a denitrated product. Although the temperature of the substance being treated is substantially constant while the denitration reaction proceeds, the temperature of the substance is again increased after substantial completion of the denitration reaction (at point D). A residual damp nitrate partially remaining in the heating vessel is then decomposed so that the denitration reaction further proceeds. After completion of the denitration reaction of the residual nitrate, the substance being treated is completely converted into oxide (at point E), and the temperature of the substance is further increased. The application of microwaves is stopped at this time and the denitrated product (UO.sub.3) is taken out of the heating vessel.
However, as it is quite difficult to precisely measure the temperature of the substance being treated under the application of microwaves, this makes it difficult to precisely determine a completion point of the denitration reaction (at point E). As a result, there are always concerns that the application of microwaves is conducted over its limit to partially produce a superheated product (U.sub.3 O.sub.8) of the oxide (UO.sub.3) or that the application of microwaves is stopped at a time when the denitration of the substance being treated is not completed. When the superheated product U.sub.3 O.sub.8 is produced, the temperature increases in an accelerative manner since U.sub.3 O.sub.8 is larger than UO.sub.3 in microwave absorption efficiency, so that adjacent UO.sub.3 is converted into U.sub.3 O.sub.8 one after another. When UO.sub.3 is converted into U.sub.3 O.sub.8, the temperature of the substance being treated increases so much that damage to the instruments occurrs and therefore, it is necessary to stop the application of the microwaves at a time when U.sub.3 O.sub.8 is produced. In the case where the application of microwaves is stopped at a point between point D and point E, there remains an undenitrated portion. Namely, in the case where the application of microwaves is stopped at a time when the denitration of the substance being treated is not completed, the denitrated product can not be effectivley taken out of the heating vessel due to the presence of the residual damp nitrate.
As a method for detecting a heating condition of the substance being treated under the application of microwaves, there is a method for measuring reflected waves of the applied microwaves from the substance being treated. However, in the case where the microwave heating is conducted by using the conventional apparatus as shown in FIG. 5, the heating vessel 1 rotating in the oven 2 disturbs the microwave distribution in the oven which largely affects the reflection of the microwaves, so that, as shown in a graph in FIG. 7, a difference between the maximum value and the minimum value of the reflected waves becomes large. In addition, in the case where the oven 2 has such a large size that the length of its one side is equal to a total length of several wave-lengths of the applied microwaves, the area of the inner surface of the oven 2 is larger than the surface area of the substance S being treated (surface area of the heating vessel 1) so that the inner surface of the oven largely affects the reflected waves of the microwave to make the reflected waves of the microwave coming from the substance S ambiguous.
Although the apparatus itself for measuring the reflected waves of microwave has already been developed, it is impossible, due to the above-mentioned reasons, to precisely measure the reflected waves coming from the substance being treated even when the reflected wave measuring apparatus is employed in such a conventional denitration apparatus by microwave heating as shown in FIG. 5.